In a direct injection assembly of the common-rail type, it is known to use a high-pressure pump which receives a fuel flow from a tank by means of a low-pressure pump and feeds the fuel to a common-rail hydraulically connected to a plurality of injectors. As known, in such a direct injection assembly of the common-rail type, the pressure of the fuel inside the common-rail must be constantly controlled according to the driving point either by varying the instantaneous delivery of the high-pressure pump or by constantly feeding an excess of fuel to the common-rail and by discharging the fuel in excess from the common-rail itself by means of an adjustment valve. Generally, the solution of varying the instantaneous delivery of the high-pressure pump is preferred, because this solution displays a much better energy efficiency and does not result in overheating the fuel inside the tank.
In order to vary the instantaneous fuel flow of the high-pressure pump, it has been proposed, for example in EP-A-0481964, which is incorporated by reference, to use a varying delivery high-pressure pump able to feed to the common-rail only the amount of fuel needed to keep the pressure of the fuel inside the common-rail equal to a desired value.
Specifically, the high-pressure pump proposed in EP-A-0481964 is provided with an electromagnetic actuator able to vary instant-by-instant the delivery of the high-pressure pump itself by varying the closing instant of an intake valve of the high-pressure pump.
Alternatively, in order to vary the instantaneous delivery of the high-pressure pump, it has been proposed instead to insert an adjustment device including a continuously varying hydraulic resistor, upstream of the pumping chamber, which hydraulic resistor is controlled according to the required pressure in the common-rail.
Both the above-described solutions for varying the instantaneous delivery of the high-pressure pump are mechanically complex and do not allow to adjust the instantaneous delivery of the high-pressure pump with high accuracy required in principle. Furthermore, in the delivery adjustment device, the varying section hydraulic resistor includes a relatively small introduction section in case of low deliveries such as to determine a local pressure drop (local load drop) which may compromise the correct operation of an intake valve which adjusts the fuel inlet into a pumping chamber of the high-pressure pump.
For this reason, it has been proposed, e.g., which is incorporated by reference, in EP-A-1612402, a solution which includes the use of a high-pressure pump including a number of pumping elements actuated in a reciprocating motion by means of corresponding intake and delivery strokes, and in which each pumping element is provided with a relative intake valve in communication with an intake pipe fed by the low pressure pump. On the intake pipe a shut-off valve is arranged for adjusting the instantaneous delivery of fuel fed to the high-pressure pump; in other words, the shut-off valve is a known valve of the open/closed (ON/OFF) type which is driven by modifying the ratio between the duration of the opening time and the duration of the closing time so as to vary the instantaneous delivery of fuel fed to the high-pressure pump. By operating in this manner, a shut-off valve may be used in which the introduction section is sufficiently large to avoid an appreciable local pressure drop (local load drop).
When the shut-off valve of the open/closed (ON/OFF) type is closed, a hydraulic phenomenon known as “water hammer” occurs in the intake pipe. The “water hammer” occurs in the intake pipe when the fuel flow therein is either interrupted by closing the shut-off valve or, on the other hand, when the shut-off valve is closed and opened in an essentially short interval of time. The “water hammer” consists in an overpressure which originates in proximity of the shut-off valve due to the impact of the moving fuel against a shutter of the shut-off valve and propagates along the intake pipe, resulting in an increase of noise generated by the injection assembly. The generated overpressure, in addition to depending on the dimensions of the intake pipe, i.e., on the length and the diameter of the intake pipe, also depends on the speed and density of the fluid and, above all, depends on the closing and opening time of the shut-off valve of the open/closed (ON/OFF) type, which is essentially reduced, i.e., in the order of 0.5*10−3 sec.